1. Field of the Invention
The electronic nutating disc flow meter (sometimes hereinafter flow meter) is suitable for measuring different liquids with varying viscosities. This flow meter is suitable for measuring thin viscosity liquids having a centipoise of less than 9, such as water. However, this flow meter is best suited for measuring medium viscosity liquids of 10 to 450 centipoise. This flow meter is often used to measure agricultural chemicals including, but not limited to, liquid herbicides, liquid pesticides and liquid fertilizers and may be field calibrated for greater accuracy with specific liquids. This flow meter includes a microprocessor to correct for the nonlinearity of fluid flow.
2. Description of the Prior Art
Fluid meters typically have some mechanical drive element immersed in the flow path of the fluid, such as a nutating disc, a paddlewheel or a turbine. Fluid flowing through the meter causes movement of the mechanical drive element immersed in the flow. This mechanical movement or rotational speed is generally proportional to the rate-of-flow of the fluid, so that by sensing the rotational speed a fluid meter can determine flow rate and/or volume. Unfortunately, the rotational speed of the mechanical drive element is not directly proportional to the flow. For example, doubling the flow rate will not double the rotational speed of the nutating disc in the flow meter. Furthermore, the rate of change is not constant throughout the useful flow range of the electronic nutating disc flow meter. This nonlinearity is well known to those skilled in the art. A graph known as a "K-Factor" plot clearly indicates the curved-line relationship between fluid flow rate and rotational speed of the mechanical drive element. Most importantly, nonlinearity becomes more pronounced for viscous fluids which creates instrument error.
Various attempts have been made to correct for this nonlinearity and thus improve the ultimate accuracy of fluid meters. See for example, U.S. Pat. No. 4,306,457; 4,581,946; 3,965,341; 4,593,365 and 4,885,943.
The present invention is typically installed in agricultural fluid delivery systems; however, it is not limited to agricultural applications. Scienco, Inc. of Memphis, Tenn., currently sells an electronic nutating disc flow meter, the model SEM-20. The Scienco meter is typically mounted at or near the pump in an agricultural fluid delivery system. The Fill-Rite Division of the Tuthill Corporation of Fort Wayne, Ind., also sells an electronic nutating disc flow meter, the model 810 digital meter. The Fill-Rite meter is typically mounted in-line and adjacent to the dispensing nozzle in an agricultural fluid delivery system. The Scienco and the Fill-Rite meters have a single calibration curve which is defined by a single point. Both of these products should be field calibrated to ensure accuracy when dealing with viscous agricultural fluids.
Both the Scienco and the Fill-Rite meters use one or more magnets which are immersed in the fluid stream and which are rotated by the nutating disc. One or more reed switches are isolated from the fluid flow and close a circuit each time the magnet rotates by the reed switch. The differences between the Scienco and the Fill-Rite meters and the present invention are discussed in greater depth in the Information Disclosure Statement filed concurrently herewith.